If you' re referring to BBM, you're referring to one category: Group Messaging. Not, its not the same as Instant Messaging which is a one on one conversation.
Instant messaging: GTalk, Yahoo Messenger, AIM, eBuddy, fringo, Palringo, Nimbuzz, Skype
Group Messaging: Kik Messenger (Blackberry is suing them), Beluga, KakaoTalk, WhatsApp, QQZone.
Group Messengers for Android are actually both for iPhone and Android, since they also have iPhone equivalents. So yeah, you can chat with your friends on IPhone on the same manner and join their groups.
KakaoTalk is the most popular of the group but it's also Korean and you can bet the sudden explosion of Korean Androids have something to do with this (you know, like the Galaxy S and Optimus One). Its popularity is extending to Japan, China and Southeast Asia. But unless you're Korean and want to chat with the folks back home, it probably isn't relevant to you.
QQ has been called the Facebook of China, and QQZone is like chat between QQ members. Not relevant for US folks but it does show Chinese social networking services are dead serious with Android. One of the reasons why I see many mainland Chinese outside of the US carry an iPhone and increasingly Android is to have particular access to this service to chat with the folks back home.
So that leaves you with Kik, WhatsApp and Beluga. There can be more, but so far, these three are the most popular.